1. Technical Field
This invention relates generally to a system for controlling a switched reluctance (SR) motor, and more particularly, to a method and apparatus for reducing acoustic noise generated by an SR electric motor.
2. Discussion of the Related Art
Switched Reluctance Machines (SRMs) have been the subject of increased investigation due to their many advantages, which makes them suitable for use in a wide variety of situations. A SR machine operates on the basis of varying reluctance in its several magnetic circuits. In particular, such machines are generally doubly salient--that is, they have teeth or poles on both the stator and the rotor. The stator poles have windings wound therearound which form machine phases of the motor. In a common configuration, stator windings on diametrically opposite poles are connected in series to form one machine phase.
When a machine phase is energized, the closest rotor pole pair is attracted towards the stator pole pair having the energized stator winding, thus minimizing the reluctance of the magnetic path. By energizing consecutive stator windings (i.e., machine phases) in succession, in a cyclical fashion, it is possible to develop torque, and thus rotation of the rotor in either a clockwise, or counter-clockwise direction.
One problem with the operation of conventional switched reluctance motors is acoustic noise. The noise is caused by an abrupt change in current through the phase winding when the phase transitions from an active state (i.e., current being supplied from a power supply to the phase winding), to an inactive state (i.e., when residual current existing in the winding is either returned to the power supply or dissipated). The abrupt change in the current results in forces that manifests itself in acoustic noise. One approach taken in the art at reducing the acoustic noise involves a two stage decay of the residual current to lessen the abruptness in the change in current. During a first stage, which commences immediately after the phase is switched off, the residual current is decayed using a zero voltage decay technique (i.e., the current circulates through the winding, a switch, and a diode) for a duration equal to one-half the resonant time period of the motor. During the second stage, the remaining current is decayed to zero using a forced commutation technique (e.g., for a two-switch drive topology, the current circulates through both diodes and back through the power supply). A disadvantage with this approach is that in each stage, the current decays "naturally" and, further, the duration of each stage is the same for all the phases of the motor. While partially effective at reducing acoustic noise, the lack of control over the decay profile leaves substantial room for improvement in reducing acoustic noise.
Another approach taken in the art includes the "control" of the residual current from the start of the inactive part of the phase until the phase current is decayed to substantially zero. A disadvantage of this approach is that it may be required or desired to decay the current to zero more quickly than possible with a controlled approach over the entire inactive part of the phase.
Accordingly, there is a need to provide an improved system for controlling a switched reluctance machine that minimizes or eliminates one or more of the problems set forth above.